Update post-review to work with the Review Board 1.5 API.

This updates post-review to use the new API in Review Board 1.5 when
available. The deprecated API won't be used in this case. However, if
the new API is not available, we fall back on the deprecated API in order
to continue supporting older installations.

This will require some fixes/enhancements to the API that are going in to
1.5.2. The API checks will be updated to check for the version once that
release has been done.

This has feature parity with the old API.

Posted several review requests, updated diffs, set various fields, published,
and reopened review requests.

Verified that cookies were being successfully loaded and saved. Making a subsequent
call to post-review didn't require a re-login.

Using --username by itself prompted for a password and then tried to make use of that
password. When I gave it valid credentials, the request went through. When I continued
to give it bogus credentials, it kept re-asking, and eventually failed, just like it
used to.

Providing both --username= and --password= on the command line caused it to successfully
log in.

Tested that updating review requests works on older Python versions.

Tested that --submit-as continues to work fine.